Metal parts, for example, those used in jet engines are frequently required to meet very precise tolerances. Damage to metal parts during use or during machining where a part is overmachined can prevent the part from falling within the set tolerances and require that the part be repaired or replaced. To repair such damage, new metal must be brazed to the surface of the part. This can either fill a crack or build up the surface of the part.
The new metal needs to have a composition similar to the base metal. Thus, braze fillers are combined with powder base metal to provide a composition which brazes to the base metal at a temperature lower than the melting point or softening point of the base metal. If the part is overheated, its structural and dimensional integrity can be jeopardized.
Typically, this has been done by forming a slurry which includes the powdered base metal, powdered diffusion braze alloy and a binder. Braze alloys generally include boron which acts as a melting point suppressant. These systems provide acceptable results. However, well defined geometries needed for some repairs are very difficult to obtain. In part, this is due to the high concentration (generally 50%) of diffusion braze alloy. This caused the repair metal to flow during processing. Generally, the maximum thickness of a repair is 0.015 inch.
Further, slurries such as these are difficult to use. The binder system must be initially mixed. Then the precise amount of base metal and diffusion braze filler must be combined. This has a very limited shelf life. It cannot be mass produced for sale and subsequent use. It must be prepared by the actual user which creates the potential problem of human error.
Also, the boron added to suppress the melting point attacks and weakens the base metal. Due to the high concentration of braze alloy (which contains boron) in these repair slurries, the braze alloy can melt and puddle at the surface of the base metal. This allows the boron to attack the surface metal and significantly weaken it. Because of this, a part usually can only be repaired with such a slurry 1-3 times. After that, the part had to be scrapped. If a part required a build up of more than 0.045 inch, this would require more than three repairs, and should not be feasible due to boron attack on the base metal exceeding allowable limits.
Also, with oxygen sensitive alloys such as those that include titanium, aluminum, hafnium, and chromium, heating above 800.degree. F. can cause oxide formation. Most braze furnaces are designed to either operate in a vacuum or in a hydrogen atmosphere. However, there is frequently a trace amount of oxygen remaining in the furnace that can react with these metals. To avoid this problem, such alloys are nickel coated or fluoride ion cleaned prior to base metal repair. This nickel precoating is undesirable simply because it requires an extra step or even two extra steps frequently requiring masking of portions that are not to be nickel coated. Fluoride ion cleaning is undesirable as it requires additional processing steps including an expensive furnace operation.